The present invention relates to energy storage systems which can reduce the gearbox stress and improve quality of output energy and power. Wind power generation has been growing very aggressively in the recent years. The U.S. Department of Energy has an initiative to further increase wind energy's contribution to 20% of the U.S. electricity supply by 2030. To meet this demand in capacity, variable speed wind turbine generators have been developed. These generators usually employ sizable blades on a tower which is connected to a gearbox. The gearbox is connected by a rotor shaft to a generator, which in turn interfaces with a power converter. The converter exports the output power of the generator to the grid. The American Wind Energy Association's Operation and Maintenance Working Group reported that 25% of wind turbine failures are caused by the gearbox and generator, and 20% are caused by the power converter. Thus, there has developed a need for a wind energy power conversion system with an extended operating life of the gearbox and improved stability of the power converter.
Incumbent wind turbines may suffer from short-term and long-term variations in power output. The short-term variations in power output are mainly due to the variable wind speed. Moreover, the short-term power fluctuations can be directly converted to torque fluctuations on the rotor shaft because of the substantial inertia of sizable blades, thereby creating a substantial stress on the turbine gearbox. Furthermore, conventional wind turbines may produce long-term fluctuations in power, because the power grid load typically peaks in late afternoons when the wind speed is lower. Thus, there has developed a need for a wind energy power conversion system with an improved quality of output energy and power.